Lubricants for pressing transition metal-rare earth powder to be sintered

ABSTRACT

An organometallic compound useful as a lubricant is admixed with a magnetic transition metal-rare earth alloy powder and the mixture is pressed to form a green body. The green body is heated to decompose the organometallic compound and then sintered. The organometallic compound decomposes at a temperature below 500*C. to yield a metal vapor which deposits metal in the body and a non-metal specie which diffuses out of the body.

United States Patent [191 Smeggil LUBRICANTS FOR PRESSING TRANSITION METAL-RARE EARTH POWDER TO BE SINTERED [75] Inventor: John G. Smeggil, Elnora, NY.

[73] Assignee: General Electric Company,

' Schenectady, NY.

[22] Filed: June 22, 1973 [21] Appl. No.: 372,689

[52] US. Cl 148/105, 75/0.5 BA, 1l7/107.2,

148/103 [51] Int. Cl. H011 1/02 [58] Field of Search. 148/105, 31.57, 101;

75/211; 29/192 CP; 117/100 M, 107.2 R; 264/DIG. 58, 111; 252/46.4, 49.7

[56] References Cited I UNITED STATES PATENTS 3,342,587 9/1967 Goodrich et a1. 117/100 M 3,373,018 3/1968 Oxley et a1 3,615,914 10/1971 Becker 148/101 3,684,593 8/1972 Benz et a1. 148/3157 3,728,110 4/1973 Klar et a1. 75/211 Dec, 10, 1974 3,511,683 5/1970 Espenscheid et al 117/100 M OTHER PUBLICATIONS Harwood, J Industrial Applications of the Organometallic Compounds, New York, 1963, pp. 339, 384, 385, 430, 247, &'438.

Schwartzkopf, P., Powder Metallurgy, New York, 1947, pp. 44-45.

Primary ExaminerWalter R. Satterfield Attorney, Agent, or Firm-Jane M. Binkowski; Joseph T. Cohen; Jerome C. Squillaro 57 ABSTRACT An organometallic compound useful as a lubricant is 3 Claims, N0 Drawings LUBRICANTS FOR PRESSING TRANSITION METAL-RARE EARTH POWDER TO BE SINTERED The present invention relates generally to the art of making magnets, and more particularly, it is concerned with lubricants for pressing magnetic transition metalrare earth powders wherein the resulting pressed bodies are to be sintered and to novel. magnetic sintered products.

Magnetic properties of bulk magnetic materials having large magnetocrystalline anisotropies can be enhanced by reducing them to powders. particularly those having an average particle size of less than microns. However, in such finely divided form these materials are unstable in air and their magnetic properties deteriorate after a shortperiod of time.

The art has used sintering to produce magnets with substantially stable properties from these powders. Generally, such a process comprises die-pressing the powder toform a green body and sintering the body at high temperatures in an inert atmosphere to produce a densified product preferably having a closed pore structure to protect it from reaction with the atmosphere.

A problem arises, however, in die-pressing the powder to form a green body. A lubricant is needed to keep the die from freezing up in the pressing operation. The art has used a lubricant of organic composition which is normally composed at least in a significant part of very long chain organic compounds that provide the desired inertnesswith respect to the powder being pressed and lubricity which is needed to prevent sticking and proper formation of the green body. Some of the lubricant will be entrained in the pressed material, e.g., the green body, which is then sintered, at substantial temperatures, generally at least 900C. for a significant period of'time, usually at least one hour. In the course of this thermal treatment the organic compounds comprising the lubricant decompose and because of the initial long chain structure yield substantial amounts of either free elemental carbon and/or compounds containing carbon and possibly other elements, e.g., oxygen primarily, in a form which will react with and subsequently degrade the sintered product and diminish its magnetic properties significantly.

The present invention overcomes this problem and provides lubricants which will not only have no significant deteriorating effect on the sintered product but which can be selected to enhance the magnetic properties of the sintered product. In accordance with the present invention, an organometallic compound is used as the lubricant for diepressing magnetic transition metal-rare earth alloy powders.

Briefly stated, the process of the present invention comprises admixing an organometallic compound and particles of a transition metal-rare earth alloy to form a substantially intimate mixture, said organometallic compound decomposing at a temperature below 500C, pressing said mixture to form a green body, heating said green body to decompose said organometallic compound to yield a metal vapor and a nonmetal specie of decomposition, said metal vapor depositing metal in said body and said non-metal specie of decomposition diffusing out of said body, and sintering said body at a temperature of at least 900C. to produce a sintered product having a density of at least 87 percent of theoretical.

The organometallic compound of the present invention is a lubricant for the powder being pressed and it is inert with respect to the powder being pressed. Specifically, the organometallic compound is a solid or liquid at room temperature and it is air-stable at room temperature. It thermally decomposes at a temperature below 500C. to yield a metal vapor and a non-metal specie or species of decomposition, at least one of which is organic, and which are stable to interaction with the. pressed transition metal-rare earth alloy powders at the temperatures needed for this decomposition to occur. Specifically, the decomposition of the organometallic compound below 500C. produces a metal vapor which condenses metal on the exposed surfaces of the pressed powder and also produces an organic specie, or nonmetal species at least one of which is organic, which diffuse out of the body. No water vapor or oxygen is present to degrade the magnetic propertiesof the pressed powder.

The particular metal deposited by the decomposition reaction can be chosen to fulfill a number of purposes. For example, the metal remanent can be inert and not partake or interact in any significant manner, either chemically or magnetically, with the transition metalrare earth alloymaterial, e.g. Zr. Or else the metal can be chosen to coat the transition metal rareearth alloy powders and offer protection against later oxidation. Or the metal can be chosen to change the chemical composition of the transition metal-rare earth alloy ma terial, e.g., Co deposited from cobalt acetylacetonate and cobalt-rare earth alloy pressedmaterial in some desired manner.

Generally, the organometallic compound is used in an amount ranging from about 0.5 to 10 percent by weight of the particles of transition metal-rare earth alloy to be pressed. The specific amount of organometallic compound depends largely on the lubricating characteristics of the organometallic on the particular properties desired in the final sintered product as imparted by the particular metal deposited by the decomposition reaction. Generally, such a metal is deposited in the sintered product in an amount ranging from a detectable amount to about 2 percent by weight of the transition metal-rare earth alloy.

in the present process a magnetic transition metalrare earth alloy, e.g., TRE, where T is a transition metal and RE is a rare earth metal, is used in particle form. The transition metal is selected from the group consisting of cobalt, iron, nickel, manganese and alloys thereof.

The rare earth metals useful in the present process are the 15 elements of the lanthanide series having atomic numbers 57 to 71 inclusive. The element yttrium (atomic number 39) is commonly included in this group of metals and, in this specification, is considered a rare earth metal. A plurality of rare earth metals can also be used to form the present intermetallic compounds which, for example may be ternary, quartenary or which may contain an even greater number of rare earth metals as desired. Mischmetal, anabundant common alloy of rare earth metals, is particularly advantageous.

Representative of the cobalt-rare earth compounds useful in the present invention are cobalt-cerium, cobalt-praseodymium, cobalt-neodymium, cobaltpromethium, cobalt-samarium, cobalt-europium, cobalt-gadolinium, cobalt-erbium, cobalt-thulium, co-

balt-ytterbium, cobalt-lutecium, cobalt-yttrium, cobaltlanthanum and cobalt-mischmetal. Examples of specific ternary compounds include cobalt-ceriumpraseodymium, cobalt-yttrium-praseodymium, and cobalt-praseodymium-mischmetal.

Transition metal-rare earth intermetallic alloys or compounds exist in a variety of phases and each phase may vary in composition. A material substantially comprised of the T RE single phase is particularly preferred in the present invention since this phase has-shown the most desirable combination of magnetic properties.

The transition metal-rare earth compound or alloy of the present process can be prepared by a number of methods. For example, it can be prepared by melting the transition metal and rare earth metal together in the proper amounts under a substantially inert atmosphere such as argon and allowing the melt to solidify.

The alloy can be converted to particulate form in a conventional manner. For-example, it can be crushed to a coarse size and then pulverized to a finer form by, for example, fluid energy milling in a substantially inert atmosphere. Alternatively, the powder can be produced initially by a reduction-diffusion process as set forth in copending application Ser. No. 172,290, filed on Aug. 16, l97l in the name of Robert E. Cech. Also, in some instances, it may be desirable to grind sintered compacts of these powders to a desired particle size.

The particle size of the transition metal-rare earth alloy used in the present process may vary. it can be in as finely divided a form as desired. For best magnetic properties, average particle size will range from about l-micron or less to about microns. Larger sized particles can be used, but as the particle size is increased, the maximum coercive force obtainable is lower because the coercive force decreases with increasing particle size.

In the present invention, the organometallic compound can be a solid. or a liquid at room temperature, and it should be admixed with the TRE alloy powder to produce a substantially intimate mixture. Such a mixture is necessary so that when it is pressed the organometallic lubricant is available for contact with the pressing or die surfaces to prevent sticking or freezing up of the die during the pressing operation.

The mixture can be pressed into a green body of the desired size and density by any of a number of techniques such as hydrostatic pressing or methods employing steel or other metallic dies. Preferably, the mixture is pressed in an aligning magnetizing field to magnetically align the particles along a desired axis, or if desired, the mixture may be pressed after magnetically aligning the particles. The greater the magnetic alignment of the particles, the better are the resulting magnetic properties. Preferably also, pressing is carried out to produce a green body with as high a density as possible, since the higher its density, the greater the sintering rate. Green bodies having a density of about 40 percent or higher of theoretical are preferred. The green body is heated to decompose the organometallic compound and sintered to produce a sintered product wherein the pores are substantially interconnecting. This is preferably carried out in a single step technique. Specifically, as the green body is heated to sintering temperature, generally at a rate ranging from about 50C. to 150C. per minute, the organometallic compound decomposes at a temperature below 500C. and the non-metal specie or species difnon- fuse therefrom. The green body is then sintered to produce a sintered body wherein the pores are substantially non-interconnecting. The sintering temperature is at least 900C. and usually about l,050C. or higher and depends largely on the particular transition metalrare earth alloy material being sintered. For example, a sintering temperature of 1,100C. is suitable for a Co Sm alloy. A sintered body having a density of at least about 87 percent of theoretical is generally one wherein the pores are substantially noninterconnecting and this is determinable by standard metallographic techniques. Such non-interconnectivity stabilizes the magnetic properties of the. product because the interior of the sintered product or magnet is protected against exposure to the ambient atmosphere.

The decomposition of the organometallic compound is carried out in an atmosphere in which the reactants are inert, e.g., an atmosphere in which the green body is inert, such as argon, and sintering is also carried out in a substantially inert atmosphere such as argon. Upon completion of sintering, the product is preferably cooled to room temperature in a substantially inert atmosphere. 7

In the present process, there are a number of organometallic'compounds which are useful as lubricants and which decompose at temperatures below 500C. Typical of these is phenylcopper, C H Cu, which thermally degrades according to the following reaction:

There are a number of advantages to the use of phenylcopper and other organometallics whichdecompose in a similar manner. One advantage'is that the low temperature at which this organometallic decomposes will not affect the magnetic properties ofthe transition met- I al-rare earth alloy powder. Another advantage is that the organometallic decomposition reaction is relatively clean and yields an organic specie which readily diffuses out of the body.

Table l lists a partial series of organometallic compounds suitable for the present process.

Bi Trimethyl bismuth, Bi(CH )3 Au Diethyl gold bromide, ((C;,H,,) AuBi),

Pb Tetraethyl lead, Pb(C,H

Mn Dicyclopentadienyl manganese, (C H hMn Ti Dicyclopentadienyl titanium, (C,,H Tl

In addition to the above organometallics listed in Table I, there are a number of trifluoroacetylacetonates, hexafluoroacetylacetonates of various metals, e.g., Zn and Zr, which could function as lubricants.

The sintered products of the present invention, when magnetized, are useful as magnets. These magnets are substantially stable in air and have a wide variety of uses. For example, they are useful in telephones, elecexamples.

EXAMPLE l A sintered body of compacted CoSm alloy powder, prepared substantially as set forth in U.S. Pat. No. 3,655,464, was ground to a powder using ajaw crusher and a jet mill. The alloy powder was comprised substantially of Co Sm phase and a minor amount of Co Sm phase and ranged in size from +44 to 77 microns. Copper acetylacetonate, present as the hydrate Cu(CH COCHCOCH .2H O was admixed with the alloy powder in an amount of 0.5 percent by weight of the alloy powder to form a substantially intimate mixture. The mixture was placed in a rubber tube which was then sealed and hydrostatically pressed under a pressure of 200,000 psi at room temperature for about minutes to form a green body. The rubber tube was then readily peeled away from the resulting green body and no sticking occurred between the green body and the inner surfaces of the rubber tube.

The green body was sintered at 1,120C in argon for 1 hour and cooled to room temperature in argon. The resulting compact was magnetized at room temperature in a magnetizing field of 60,000 oersteds. The resulting magnet displayed useful magnetic properties as demonstrated by using it to lift a number of small metallic objects. The sintered body had a density of at least about 87 percent of theoretical.

The sintered compact was sliced and polished using standard metallographic techniques. The polished slices were examined under an optical microscope. No appreciable amount of oxide which would result from interaction between the decomposition products of the copper acetylacetonate and the cobalt-Samarium alloy powder was detected.

EXAMPLE 2 The cobalt-Samarium alloy powder used in this example was the same as that used in Example 1. Copper acetylacetonate in an amount of almost 10 percent by weight of the alloy powder was admixed therewith to form a substantially intimate mixture. The mixture was placed in a steel die press and pressed at room temperature under a pressure of about 300,000 psi for about 2 minutes. The resulting pressed body was easily slipped EXAMPLE 3 The procedure usedin this example was the same as that set forth in Example 2 except that no organometallic compound was used. The resulting green body adhered significantly to the surfaces of the die press and was dislodged therefrom with difficulty resulting in a significant loss of alloy powder.

In copending U.S. Pat. application Ser. No. 372,690 entitled Fabrication of Matrix Bonded Transition MetaLRare Earth Alloy Magnets" filed of even date herewith in the names of Richard J. Charles and John G. Smeggil there is disclosed a process for producing an air-stable porous magnetic compact which comprises admixing particles of a transition metal-rare earth alloy with an organometallic compound which decomposes at a temperature below 500C. and pressing the mixture to form a green body. The green body is heated to decompose the organometallic compound to produce a non-metallic product and a metal vapor. The metal vapor deposits an interconnecting continuous coating of metal on the exposed surfaces of the pressed alloy particles thereby preventing penetration by the atmosphere, and the non-metallic product is outgassed from the body leaving the resulting coated compact porous.

The above cited application is, by reference, made part of the disclosure of the present application.

What is claimed is:

1. A process for producing a sintered product of compacted particulate cobalt-rare earth alloy with improved magnetic stability which comprises providing particles of cobalt-rare earth alloy having an average size of up to about l0 microns, providing a lubricating organometallic compound which is inert to said alloy particles and which at room temperature is air-stable and a solid or liquid and which at a temperature below 500C completely decomposes and yields only products of decomposition consisting of gaseous nonmetallic product and a metal vapor, admixing said particles of cobalt-rare earth alloy and an amount of said organometallic compound ranging from about 0.5 percent to about 10 percent by weight of said alloy particles, diepressing the resulting mixture to form a green body, heating said green body at a temperature below 500C completely decomposing said compound and producing said gaseous product of decomposition and said metal vapor, said gaseous product of decomposition completely diffusing out of said body and said metal vapor condensing metal on said body, and then sintering said body at a temperature of at least 900C to a density of at least about 87 percent of theoretical, said condensed metal having no significant deteriorating effect on the magnetic properties of said sintered product, said decomposition and sintering being carried out in an inert atmosphere.

2. A process according to claim 1 wherein said alloy is a cobalt-samarium alloy.

3. A process according to claim 1 wherein said organometallic compound is copper acetylacetonate. 

1. A PROCESS FOR PRODUCING A SINTERED PRODUCT OF COMPACTED PARTICULATE COBALT-RARE EARTH ALLOY WITH IMPROVED MAGNETIC STABILITY WHICH COMPRISES PROVIDING PARTICLES OF COBALT-RARE EARTH ALLOY HAVING AN AVERAGE SIZE OF UP TO ABOUT 10 MICRONS, PROVIDING A LUBRICATING ORGANOMETALLIC COMPOUND WHICH IS INERT TO SAID ALLOY PARTICLES AND WHICH AT ROOM TEMPERATURE IS AIR-STABLE AND A SOLID OR LIQUID AND WHICH AT A TEMPERATURE BELOW 500*C COMPLETELY DECOMPOSES AND YIELDS ONLY PRODUCTS OF DECOMPOSITION CONSISTING OF GASEOUS NON-METALLIC PRODUCT AND A METAL VAPOR, ADMIXING SAID PARTICLES OF COBALT-RARE EARTH ALLOY AND AN AMOUNT OF SAID ORGANOMETALLIC COMPOUND RANGING FROM ABOUT 0.5 PERCENT TO ABOUT 10 PERCENT BY WEIGHT OF SAID ALLOY PARTICLES, DIEPRESSING THE RESULTING MIXTURE TO FORM A GREEN BODY, HEATING SAID GREEN BODY AT A TEMPERATURE BELOW 500*C COMPLETELY DECOMPOSING SAID COMPOUND AND PRODUCING SAID GASEOUS PRODUCT OF DECOMPOSITION AND SAID METAL VAPOR, SAID GASEOUS PRODUCT OF DECOMPOSITION COMPLETELY DIFFUSING OUT OF SAID BODY AND SAID METAL VAPOR CONDENSING METAL ON SAID BODY, AND THEN SINTERING SAID BODY AT A TEMPERATURE OF AT LEAST 900*C TO A DENSITY OF AT LEAST ABOUT 87 PERCENT OF THEORETICAL, SAID CONDENSED METAL HAVING NO SIGNIFICANT DETERIORATING EFFECT ON THE MAGNETIC PROPERTIES OF SAID SINTERED PRODUCT, SAID DECOMPOSITION AND SINTERING BEING CARRIED OUT IN AN INERT ATMOSPHERE.
 2. A process according to claim 1 wherein said alloy is a cobalt-samarium alloy.
 3. A process according to claim 1 wherein said organometallic compound is copper acetylacetonate. 